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Regardless of make or model, the most likely cause of brake seizure after a prolonged period of parking with the brake applied is 'rusting' of the lining to the brake drum due to moisture.
The parking air-brake mechanism operates against a very strong spring which actually applies the brake - the air acts against the spring to release the brake. If the seals are seized the spring won't release, and if the seals are perished the air will leak.
Make sure that the parking brake mechanism and linkage are releasing (check that there is movement when you apply full air pressure to the cylinder - it might be seized).
Using a box spanner to 'wind-off' the spring pressure might ease the pressure on the linings sufficiently to release the brake when you try to drive the vehicle.
If the linkage isn't moving you might need to remove the actuator from the brake backplate (note do not attempt to dismantle the actuator without first winding off the spring) - failure to do so is likely to result in serious injury! Be warned.
Be aware that if you do 'wind off' the spring brakes you will have no parking brake should you manage to unseize the brakes! You will have to stop the vehicle in gear before releasing the footbrake or have suitably placed chocks to prevent the vehicle rolling away!
Remember to 'unwind' the springs in the parking brake actuators before attempting to drive the vehicle on the highway. Apart from being dangerous, it is illegal!
If you don't fully understand the above instructions - seek help from an experienced mechanic familiar with your type of vehicle. Some spring brakes need a bolt to be inserted to release the spring pressure, others have the bolt fixed into the cylinder, requiring just a box spanner to 'wind-off' the pressure. There may be instructions on the actuator unit.
Tampering with a spring brake unit that isn't compressed is dangerous and can cause serious injury.
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